When a user suggests an edit, their edit is put in the Suggested Edits review queue. The edit will leave the queue under three conditions:
- If two users (three users on Stack Overflow only) vote to approve or reject the edit
- If the original poster or a diamond moderator approves or rejects the edit (their vote is binding)
- If the Community ♦ user steps in and approves or rejects the edit
The first two should be self-explanatory, as that is the purpose of the review queues. In this case, the suggested edit has been reviewed.
However, the last one can be a bit confusing, especially since there is no reason provided when Community ♦ rejects an edit. (There is a proposal concerning this here.) Community ♦ is the automatic user designed to take care of maintenance for the site, and it may approve or reject edits under any of the following conditions:
A user improves your edit from the review queue
When one is reviewing suggested edits, there are four available buttons, one of which is Improve:
When the reviewer chooses to improve the suggested edit, they also choose whether or not to approve or reject this edit. The edit will be approved by default, but the reviewer can override this approval with the check box titled "Suggested edit was helpful" at the bottom of the edit window:
If the reviewer chose to mark your edit as helpful, Community ♦ will then approve your edit and you will gain two reputation as usual. Otherwise, Community ♦ will reject your edit and you will not gain the two rep. Here, the user has marked the edit as helpful:
A user with full edit privileges saves an edit over yours
If a user with full editing privileges (note that the original poster has full editing privileges on their own posts) begins editing the post at the same time as you, then your edit will be overridden in favor of theirs, if both are saved at approximately the same time.
This is known in software as an optimistic lock. We can assume that this is an edge case, and does not happen very often. Thus, when one person starts editing a post, we do not need to lock everyone else out. While the UI attempts to avoid these situations (by prompting you with "an edit has been made to this post; click to load"), they do sometimes occur.
This sometimes appears strange to a viewer—as if Community ♦ has immediately rejected their edit, without warning, with no hesitation.
Do not be alarmed! 'Tis just concurrent modification, working its evil magic once again.
When determining if this is the cause of the rejection, keep in mind that a user gets a 5-minute grace period when editing. If they submit another edit that overrides your suggestion within this grace period, a new revision is not created, but your suggestion is still automatically rejected due to the edit conflict.